Since the internet has been popularized in recent years, demand for magnetic disk drives has increased along with the increase of shipments of personal computers. While connection can be made to the internet not only from personal computers but also from mobile terminals, it is preferred to incorporate a magnetic disk drive for handling a large capacity of information in order to attain a further convenience in the mobile terminal. Further, along with digitalization of television broadcasting, a trend of using magnetic disk drive as a video recording device is increasing. As described above, the field of application of the magnetic disk drive has been extended more and more, and decrease in the size and increase in the capacity have been demanded from users.
A longitudinal recording system has been adopted in existent magnetic disk drives. In longitudinal recording, the direction of magnetization to be recorded is in plane and adjacent magnetization is in an opposite polarity. Accordingly, a magnetization transition region is formed between adjacent recording bits for decreasing a static magnetic energy. A large width of the magnetization transition region causes an increase of the noise. In order to decrease the same, it is considered that reduction of the thickness of the magnetic film and refinement of the magnetic crystal grain size are effective. Accordingly, approaches to higher recording densities of a longitudinal recording media have mainly been directed to decreasing the volume of a micro-magnet constituting a recording bit. However, it is considered that further higher recording density in the future cannot be coped easily with the longitudinal recording medium owing to a physical limit. That is, the longitudinal recording medium causes a problem for the basic function of preserving recorded information due to thermal fluctuation phenomenon of magnetization along with refinement of the micro-magnet constituting the recording bit.
Therefore, perpendicular magnetic recording media have been developed mainly in recent years. In the perpendicular recording, since the direction of the recording magnetization is perpendicular to the film surface and no strong charge is present between adjacent recording bits, the width of the magnetization transition region does not increase as much as in the longitudinal medium. On the other hand, the perpendicular recording has a characteristic that the anti-magnetic field exerting between adjacent bits decreases as the recording density increases to keep the recording magnetization stable. Further, since a strong head magnetic field is obtained by providing a soft magnetic backing layer having a high permeability below the perpendicular recording layer, the coercivity of the perpendicular recording layer can be increased and this is one of the promising approaches for overcoming the limit of thermal demagnetization in the longitudinal recording system. Accordingly, a further increase in the density of a magnetic disk drives (HDD) is possible and application to HDD products has already been started in some fields.
A medium used in the perpendicular recording system mainly comprises a soft magnetic backing layer that assists a recording head, and a vertical magnetic recording layer for recording and possessing magnetic information. It is desirable to use, as the perpendicular magnetic recording layer, those materials having a strong perpendicular magnetic anisotropy such that the recording magnetization is arranged in a direction perpendicular to a film surface and in which each of magnetic particles is isolated magnetically for reducing noises. Specifically, granular type materials comprising a Co—Cr—Pt type alloy with addition of an oxide such as SiO2 have been studied generally. In the granular type perpendicular recording layer, since non-magnetic oxides form a grain boundary so as to surround the magnetic particle, magnetic interaction between adjacent magnetic particles is decreased. Further, since the grain boundary of the oxide suppresses the coupling between the magnetic particles, this has a feature capable of decreasing the dispersion of the particle size compared with existent Cr segregation type longitudinal recording media. The perpendicular magnetic recording medium having such a fine structure has low noise property and excellent heat stability together, for which a great expectation has been placed in the improvement of the recording density.
However, when the magnetic interaction between adjacent magnetic particles greatly decreases, each of the magnetic particles strongly tends to reverse independently to increase the dispersion of a reversed magnetic field. On the other hand, for the recording head, a study has been progressed for a head with a trailing shield in order to improve the magnetic field gradient and improve the recording resolution power. In the recording head of this type, the recording magnetic field intensity tends to lower compared with existent monopole type heads. In such a situation, improvement in the information writing performance has been demanded while ensuring low noise and excellent heat stability for the perpendicular magnetic recording medium.
For the demand for the perpendicular magnetic recording medium as described above, Japanese Patent Application No. 2004-310910 (“Patent Document 1”), for instance, proposes a medium having two or more magnetic layers for a perpendicular recording layer, at least one layer being a layer comprising Co as a main ingredient, containing Pt, and containing an oxide, and at least one the other layer being a layer comprising Co as a main ingredient, containing Cr, and not containing an oxide. With such a layer constitution of the perpendicular recording layer, refinement and magnetic isolation of magnetic particles are promoted and signal/noise ratio upon reading can be improved greatly. Further, it is described that also the thermal fluctuation resistance can be improved by improving the reverse magnetic domain nuclei forming magnetic field (—Hn) and a medium having a further excellent recording characteristic can be obtained.
Further, Japanese Patent Publication No. 2006-302426 (“Patent Document 2”) discloses a granular perpendicular recording layer comprising a Co—Cr—Pt type alloy with addition of an oxide such as SiO2 in which the oxygen concentration on the side of the boundary with an intermediate layer is increased to higher than that on the side of the surface. The intermediate layer is a layer disposed below the perpendicular recording layer and responsible to the crystal orientation property of a perpendicular recording layer and formation of a model for the granular structure. The main point of the technique is that the degree of segregation of the oxide is different depending on the growing stage of magnetic crystal grains and segregation is promoted by increasing the oxygen concentration since segregation is more difficult in the initial stage of growing than that during growing. That is, it proposes to change the oxygen concentration contained in the magnetic layer in the direction of the film thickness.
Japanese Patent Publication No. 2004-22138 (“Patent Document 3”) discloses two techniques. The first technique controls a gas pressure stepwise upon forming an intermediate layer in which a portion for the initial layer is formed at a low gas pressure and a surface portion is formed at a high gas pressure. It is described that the crystallinity and orientation property are ensured in the initial layer by a dense structure, the surface layer forms a more coarse structure than the initial layer, and a structure of a large crystal grain boundary width is formed so as to absorb gas molecules easily. In the second technique, an intermediate layer is formed in an atmosphere with addition of a micro-amount oxygen or nitrogen to a rare gas. Alternatively, the surface is exposed to a gas formed by adding a micro-amount of oxygen or nitrogen to the rare gas. It is described that this can segregate oxygen or nitrogen in the crystal grain boundary of the intermediate layer to form a formation site of a non-magnetic non-metal as a crystal grain boundary for the perpendicular recording layer formed thereon. However, when the intermediate layer is divided at least into two regions, oxygen is not added to a portion of the initial layer and oxygen is added to a portion of the surface layer, referring to the respective gas pressures, it is 10 mTorr (1.3 Pa) for the portion of the surface layer relative to 30 mTorr (4.0 Pa) for the portion of the initial stage layer, and the portion of the surface layer is formed at a low gas pressure (see Patent Document 3, FIG. 5).
In the increase of the density of the perpendicular magnetic recording medium, refinement and magnetic isolation of magnetic crystal grains are important subjects as also described for the prior art. For this purpose, it is important to prepare a model on the surface of the intermediate layer so as to promote the growing of the magnetic layer that separates into a segregation phase comprising magnetic crystal grains and the oxide. Since the crystal orientation property of the magnetic crystal grains is also controlled simultaneously in the intermediate layer, it is not allowed that the crystallinity is worsened for the formation of the model.
When isolation of the magnetic crystal grains is promoted, since the respective magnetic crystal grains behave independently to the recording magnetic field, reversal of the polarity by the magnetic head becomes difficult to remarkably lower the ease of writing. Accordingly, when the isolation of the magnetic crystal grains is promoted, the effect cannot be obtained to the utmost degree unless the writing performance is improved at the same time. Further, since it may be a worry that the scratch resistance is weakened as the magnetic crystal grains are isolated, ensuring the reliability is important for isolation. It is necessary to attempt compatibilization while finding the limit thereof.